1. Field of the Invention
The present invention relates to a lens evaluation method for evaluating resolution of a lens used for a projector, where an image light including test pattern for measuring resolution is irradiated on a screen through a projection lens to display the image of the resolution-measuring test pattern on the screen and the luminance of the displayed test pattern image is detected by an image import device using an image sensor to calculate a resolution evaluation value based on the detected luminance value, and a lens-evaluating apparatus for implementing the evaluation method.
2. Description of Related Art
Conventionally, a projector including a plurality of liquid crystal panels for modulating a plurality of colors of light in accordance with image information, a cross dichroic prism for combining the color light modulated by the respective liquid crystal panels, and a projection lens for enlarging and projecting the light combined by the prism has been used.
The projection lens used in the projector may have diverse properties such as image resolution, flare and chromatic aberration on account of variation in production process thereof. Since the diversity of the properties of the projection lens exerts influence on the image displayed by the projector, the properties of image resolution, flare and chromatic aberration are evaluated before the lens manufacturer forward the lens and before the lens is assembled into a projector.
Specifically, in order to evaluate, for instance, the resolution of the projection lens, a resolution-measuring test pattern is formed on an evaluation sheet, light is irradiated on the test pattern to introduce an image light including the test pattern into the projection lens, and the image light irradiated by the projection lens is projected on the screen. The image of the test pattern displayed on the screen is detected by an image import device using an image sensor such as a CCD (Charge Coupled Device). The image detected by the device is image-processed by a computer etc. to evaluate the resolution of the projection lens.
In the above, MTF (Modulation Transfer Function) value is generally used as a resolution evaluation value for evaluating the resolution of the lens, which can be obtained according to following formula, where the maximum value of the detected luminance value of the test-pattern image is Imax and the minimum value thereof is Imin.
MTF=(Imaxxe2x88x92Imin)/(Imax+Imin)
However, in the above arrangement, since measured luminance used for calculating the MTF value calculated by the above formula is a relative value, the MTF value may be varied according to brightness of the image.
Further, in evaluating the resolution of the projection lens, since the image light irradiated from the projection lens to be projected on the screen tends to have the strongest intensity at the central portion to be grown weaker toward the peripheral portion thereof, even when the luminance value is obtained on a plurality of portions of the image projected on the screen, the MTF value cannot be evaluated according to the same standard.
Furthermore, when the brightness of the projected image differ according to the type of the projector, the MTF value of the respective projectors cannot be compared according to the same standard.
An object of the present invention is to provide a lens evaluation method and lens-evaluating device capable of appropriately evaluating the resolution of the lens without being influenced by the type of projector and measured position.
In order to achieve an object of the present invention, a lens evaluation method according to an aspect of the present invention includes the steps of: irradiating an image light including a resolution-measuring test pattern onto a screen through a lens to display an image of the resolution-measuring test pattern on the screen; detecting a luminance of the image of the displayed test pattern by an image import device using an image sensor, a resolution evaluation value being calculated based on the detected luminance value; acquiring a background luminance value of a background part without the test pattern being formed by the image import device using the image sensor; acquiring a maximum luminance value in the test pattern image by the image import device using the image sensor; acquiring a minimum luminance value in the test pattern image by the image import device using the image sensor; and calculating the resolution evaluation value based on the background luminance value, the maximum luminance value and the minimum luminance value obtained through the respective steps.
A resolution chart measuring test pattern having a bright part and a dark part at a predetermined spatial frequency used in evaluating general optical system can be used as the resolution-measuring test pattern. The spatial frequency may be a plurality of spatial frequencies between 20 line/mm to 80 line/mm. Specifically, a parallel-line resolution chart may be used as the resolution-measuring test pattern, the spatial frequency being set as 20 line/mm, 25 line/mm, 30 line/mm, 50 line/mm and 80 line/mm, where two types of mutually orthogonal parallel-line resolution charts are used as one set to form the test pattern.
The above maximum luminance value refers to the luminance value at the brightest part in the test pattern image, the minimum luminance value refers to the luminance value at the darkest part, which can be obtained by conducting image processing for detecting luminance of an image such as pattern matching on the test pattern image taken by the image sensor.
The evaluation method of the present invention is suitably applied in an arrangement using an image import device with an image sensor having divergence between level-zero value on the output side and level-zero value on the input side so that level-zero light is not outputted from the output side even when the level-zero light is introduced into the input side thereby generating offset value. The image sensor may be CCD, MOS (Metal Oxide Semiconductor) sensor etc. The image import device may be an image data generator such as a video capturing board for converting an output of the image sensors into an image signal for computer.
According to the above arrangement, since the resolution evaluation value is calculated based on the background luminance value, the maximum luminance value and the minimum luminance value, even when the brightness of the image projected on the screen differs, the resolution evaluation value calculated based on the background luminance value, the maximum luminance value and the minimum luminance value taken at a plurality of positions can be evaluated according to the same standard by conducting correction processing by adding the background luminance value, so that the resolution of the lens can be appropriately evaluated without being influenced by the type of projector and location on the displayed image.
In the above, the resolution evaluation value (MTF) calculated by the evaluation value calculating step is represented according to following formula [1] where the background luminance value is represented as Io, the maximum luminance value is represented as Imax and the minimum luminance value is represented as Imin.
MTF=(Imaxxe2x88x92Imin)/(Io*2xe2x88x92Imaxxe2x88x92Imin)xe2x80x83xe2x80x83[1]
The formula [1] can be obtained according to the following process.
When the image of the parallel-line resolution-measuring pattern C1 as shown in FIG. 1 is detected using an image sensor and the resolution is evaluated on the luminance value of the bright part and dark part of the pattern C1 detected by the image sensor, the resolution evaluation value MTF can be given as a ratio between an input level as an input contrast ratio of the image light incident on the projection lens and an output level as an output contrast ratio of the image detected by the image sensor such as CCD camera, which can be calculated by the following formula [2].
MTF=(output level)/(input level)xe2x80x83xe2x80x83[2]
The output level in the formula [2] can be substituted by (Imaxxe2x88x92Imin), so the formula [2] can be replaced with following formula [3].
MTF=(Imaxxe2x88x92Imin)/(input level)xe2x80x83xe2x80x83[3]
On the other hand, as shown in FIG. 2, when the value of (I100%xe2x88x92Imax) is equal to the value of (Iminxe2x88x92I0%), the input level can be calculated according to the following formula [4] under the condition that the input-side I0% is the same value as the value of the output-side.
xe2x80x83(Input level)=Imax+(I100%xe2x88x92Imax)=Imax+(Iminxe2x88x92I0%)=Imax+Iminxe2x80x83xe2x80x83[4]
Accordingly, the resolution evaluation value MTF can be calculated according to following formula [5] as described in the related art section.
MTF=(Imaxxe2x88x92Imin)/(Imax+Imin)xe2x80x83xe2x80x83[5]
However, when the relationship between the input level and the output level is examined from the image acquired by the image import device using the image sensor such as CCD, offset value (I0%xe2x88x92ICCD%) is generated on the output level side as shown in FIG. 3. Accordingly, when the input level is calculated based on the formula [5], twice as much value as the offset value is added, so that the calculated input level becomes greater than an actual input level value. Further, the offset value of the image sensor such as CCD may change in accordance with the change in the background luminance value. For instance, the offset value becomes greater as the background luminance value is decreased (darkened), so that, as a result, the calculated resolution evaluation value MTF becomes smaller than the actual value, which becomes further smaller as the background luminance is decreased (darkened).
Accordingly, in order to obtain an accurate resolution evaluation value MTF, a correction processing for removing offset amount of the image import device using the image sensor such as CCD is required.
In order to remove the offset amount, the input level can be obtained according to the following formula [6] when the output-side maximum luminance value Imax, minimum luminance value Imin shown in FIG. 3, and a read value Io of the luminance on the output-side at the maximum luminance value I100% on the input-side can be determined.
(Input level)=Imaxxe2x88x92Imin+(Ioxe2x88x92Imax)*2=Io*2xe2x88x92Imaxxe2x88x92Iminxe2x80x83xe2x80x83[6]
Since the resolution evaluation value MTF becomes 1 when the spatial frequency=0, i.e. when there is no pattern, the brightness of the part having no test pattern, i.e. the luminance value of the background part should be measured. Further, the formula [6] is calculated by adding difference of the luminance value detected by the image sensor such as CCD, so that the offset amount is canceled and the obtained input level becomes a value removed with the offset amount. According to the above, the appropriate resolution evaluation value MTF removing the offset amount of the image import device using the image sensor such as CCD can be obtained according to following formula [7].
MTF=(Imaxxe2x88x92Imin)/(Io*2xe2x88x92Imaxxe2x88x92Imin)xe2x80x83xe2x80x83[7]
According to the present invention, since the resolution evaluation value MTF can be calculated according to the formula represented as [7], accurate resolution evaluation value can be obtained and the resolution of the projection lens can be appropriately evaluated without being influenced by the type of projector and location on the displayed image.
When the above-described image sensor is a charge coupled device, the background luminance value acquiring step, the maximum luminance value acquiring step and the minimum luminance value acquiring step may preferably be conducted at a part where an output of the charge coupled device in response to the luminance value is in proportional relationship.
In the image sensor such as CCD, the relationship between the image light and the luminance value lacks proportionality where the output is too bright or too dark, thus failing to obtain an appropriate luminance value. Accordingly, by providing a light adjuster such as diaphragm for adjusting brightness of the image light incident on the image sensor such as CCD, the measurement can be conducted at a portion where the linearity of the image sensor can be maintained, so that the accurate resolution evaluation value can be calculated in the evaluation calculating step.
The lens may preferably be arranged as a lens set including a plurality of light condensers disposed along an optical axis direction and has a zooming function for enlarging and contracting a projected image by changing relative position of the respective light condensers, and the background luminance value acquiring step, the maximum luminance value acquiring step and the minimum luminance value acquiring step may preferably be conducted at least for the minimum magnification and maximum magnification of the lens respectively.
When the projected image is enlarged and contracted by changing relative position of the respective light condensers of a lens set having a plurality of light-condensers disposed along an optical axis direction, the resolution evaluation value MTF may sometimes show a different value according to the enlarged projected image and the contracted projected image. Accordingly, in order to evaluate the resolution, the resolution evaluation value MTF is calculated at the minimum magnification and the maximum magnification of the lens set for evaluating the lens. By conducting such evaluation, when the lens set is installed in a projector, divergence of the resolution evaluation value MTF caused when the projected image is enlarged and contracted can be reduced in the projector.
When the image sensor is movable along the screen, the method may preferably further include the steps of: moving the image sensor along an outer periphery of the projected image projected on the screen; acquiring the peripheral image of the projected image at a predetermined position by the image import device using the image sensor while moving the image sensor; and calculating a distortion aberration of the projected image based on the peripheral image of the projected image acquired during the peripheral image acquiring step.
According to the above arrangement, since the image sensor is movable along the surface of the screen and the image sensor moving step, the peripheral image acquiring step and the distortion aberration calculating step are provided, the image sensor can be moved along the outer periphery of the projected image projected on the screen and the peripheral image can be acquired at the predetermined position by the image import device using the image sensor. Therefore, the peripheral image can be acquired at any position on the projected image and can be compared with a designed image projecting position to calculate the distortion aberration, so that the ambiguity of evaluation accuracy of conventional visual check can be eliminated and the distortion aberration can be accurately evaluated.
A check sheet formed with the test pattern may preferably include a frame portion formed adjacent to an outer periphery of a formation area of the projected image, and the image of the frame portion may preferably be acquired during the peripheral image acquiring step.
According to the above arrangement, when the check sheet includes the frame portion formed adjacent to the outer periphery of the formation area of the projected image, the profile of the frame portion can be easily and highly accurately identified from the peripheral image of the obtained frame portion by obtaining the peripheral image along the outer periphery of the frame portion formed on the screen during the peripheral image acquiring step, thereby further accurately evaluating distortion aberration.
The above method may preferably further include the steps of: calculating an input level value based on the background luminance value, the maximum luminance value and the minimum luminance value, where the background luminance value acquiring step, the maximum luminance value acquiring step, the minimum luminance value acquiring step and the input level value calculating step are conducted at a plurality of positions in the projected image; acquiring an illumination at a predetermined first position of the projected image where the background luminance value acquiring step, the maximum luminance value acquiring step, the minimum luminance value acquiring step and the input level value calculating step are conducted; and calculating an in-plane illumination of the entire projected image by calculating the illumination of a second position other than the first position based on the input level value and illumination at the first position and the input level value at the second position.
According to the above arrangement, the input level value as a relative value can be obtained by conducting the background luminance value acquiring step, the maximum luminance value acquiring step, the minimum luminance value acquiring step and the input level value calculating step at a plurality of positions in the projected image.
Further, by conducting the illumination acquiring step at the predetermined first position and the in-plane illumination calculating step, the in-plane illumination distribution can be evaluated based on the illumination at the predetermined position and the input level value.
The input level value is calculated based on the background luminance value, the maximum luminance value and the minimum luminance value and, therefore, is an evaluation value canceling the offset value generated in the image import device using an image sensor, so that the illumination at the predetermined position and the in-plane illumination of the projected image calculated based on the input level value can be evaluated in accordance with the same standard. Accordingly, the ambiguity of accuracy in conventional visual check can be eliminated and accurate in-plane illumination distribution can be evaluated.
In the above, the illumination (Le) at the second position may preferably be represented as
Le=Lo*Iie/Iioxe2x80x83xe2x80x83[8]
where the input level value at the second position is represented as lie, the input level value at the first position is represented as Iio and the illumination at the first position is represented as Lo.
According to the above arrangement, since the illumination of the second position can be obtained as a product of ratio of input level value as a relative value calculated according to the above formula [8] and the illumination of the first position as an absolute value, accurate illumination can be obtained and the in-plane illumination distribution can be evaluated by comparing the plurality of illuminations. Accordingly, more accurate in-plane illumination distribution of the projected image can be evaluated.
A lens-evaluating apparatus for evaluating a resolution of a lens according to another aspect of the present invention includes: a check sheet formed with a resolution-measuring test pattern; a light source for irradiating light on the check sheet to introduce an image light including the test pattern to the lens; a screen to which the image light irradiated by the lens is projected; an image sensor for taking an image of the test pattern displayed on the screen; an image import device for importing the image taken by the image sensor to generate an image signal; and a signal processor including a resolution evaluation value calculator that arithmetically operates the resolution evaluation value based on the image signal outputted by the image import device, in which the image sensor is provided with a light adjuster for adjusting an amount of light incident on the image sensor, the light adjuster being controlled based on a control signal from the signal processor.
The light adjuster may be an automatic diaphragm adjusting mechanism capable of being remotely operated from the signal processor.
According to the present aspect of the invention, since the light adjuster is provided, the amount of light incident on the image sensor such as CCD can be adjusted in accordance with dispersion of luminance of the image light on the screen, so that the amount of light inputted into the image sensor can be kept always constant, so that the resolution evaluation value calculated by the light-amount-adjusted image can be evaluated according to the same standard.
Further, the resolution evaluation value of the resolution evaluation value calculator can be calculated in the same manner as the lens evaluation method, by which the same function and advantages can be obtained. The resolution evaluation value calculator can be arranged as a program extended on an OS (operating system) for controlling the operation of the computer, which may include a background luminance value acquiring portion, a maximum luminance value acquiring portion, a minimum luminance value acquiring portion and an evaluation value calculating portion.
The lens evaluation apparatus according to the present invention may preferably further include an image sensor moving mechanism that moves the image sensor along a surface of the screen, the signal processor may further include: an image sensor controller for controlling movement of the image sensor along an outer periphery of the projected image projected on the screen; a peripheral image sensor for acquiring a peripheral image of the projected image at a predetermined position with the image import device using the image sensor while moving the image sensor by the image sensor controller; and a distortion aberration calculator for calculating a distortion aberration of the projected image based on the peripheral image of the projected image acquired by the peripheral image sensor.
According to the above arrangement, since the apparatus has the image sensor moving mechanism for moving the image sensor along the screen surface and the signal processor includes the image sensor controller, the peripheral image sensor and the distortion aberration calculator, the distortion aberration of the lens can be calculated by the distortion aberration calculator in the same manner as in the lens evaluation method, whereby the same function and advantage as in the above can be obtained.
In the lens-evaluating apparatus of the present invention, the check sheet may further include a frame portion formed adjacent to an outer periphery of a formation area of the projected image projected on the screen.
According to the above arrangement, since the check sheet includes a frame portion formed adjacent to the outer periphery of the formation area of the projected image projected on the screen, the image sensor controller can move the image sensor along the outer periphery of the frame portion, the peripheral image sensor can acquire the peripheral image of the frame portion at a predetermined position and the distortion aberration calculator can calculate the distortion aberration of the projected image based on the obtained peripheral image. Accordingly, the signal processor can easily obtain the peripheral image of the frame portion to calculate the distortion aberration, so that the distortion aberration can be rapidly and highly accurately evaluated.
The lens-evaluating apparatus according to the above aspect of the present invention may preferably include an illumination sensor for detecting an illumination at a predetermined first position in the projected image.
According to the above arrangement, since the illumination sensor for detecting illumination at the predetermined position in the projected image is provided, the difference of the illumination resulted by the nature of the lens can be evaluated by comparing the detected illumination for respective lens to be checked.
In the lens-evaluating apparatus, the resolution evaluation value calculator arithmetically may preferably operate an input level value based on the background luminance value, the maximum luminance value and the minimum luminance value and the input level value is acquired by the resolution evaluation value calculator at a plurality of positions in the projected image including the first position where the illumination is detected, and the signal processor may preferably include an in-plane illumination calculator for calculating an in-plane illumination of the entire projected image by calculating the illumination of a second position other than the first position based on the illumination at the first position detected by the illumination sensor, the input level value at the first position calculated by the resolution evaluation value calculator and the input level value at the second position.
According to the above arrangement, since the signal processor includes the in-plane illumination calculator, the in-plane illumination can be calculated by the in-plane illumination calculator in the same manner as in the above-described lens evaluation method, whereby the same function and advantages can be obtained.